1. Field of the Invention
This invention relates to a ceramic material based on silicon powder reaction-bonded in the presence of carbon, which material permits the passage of fluids, its use and process for its preparation.
2. Description of the Prior Art
German publication DE-A-34 40 364 discloses a material which is based on silicon powder reaction-bonded in the presence of carbon and in which silicon particles are mixed with an organic binder. A molding is formed from the mixture and is subjected to a final heat treatment during which the organic substance carbonizes and the remaining silicon skeleton containing small amounts of carbon is stabilized by reaction-bonding. It is also possible to produce a porous structure if the silicon powder is initially taken in a particle size distribution which permits an appropriately loose and permeable bed. However, the porosity of the resulting material is, in the end, always related to the size of the silicon particles used, so that open-pore structures having pore diameters in the range from 0.1 to 10 mm require the use of relatively large silicon particles. The use of such large particles results in, on the one hand, a reduction in the mechanical stability of the product and, on the other hand, a disadvantageous ratio of pore volume to total volume. Thus, this material has advantages mainly in the fine-pore range.
German publication DE-A-23 49 472, filed based on the priority of the U.S. application, Ser. No. 294,048 (applicant: Ford-Werke), discloses the preparation of a catalyst support consisting of silicon nitride and having through channels. A suspension is produced from silicon powder, a solvent and a plasticizing agent and a binder and is applied to a flat substrate of paper or textile fabric. The coated substrate is then preshaped in a corrugating apparatus, after which it is wound to form a cylindrical structure in such a way that the corrugations form gas passages through the structure. The organic material is then removed by heating in air, and finally the silicon is converted into silicon nitride in a nitriding furnace. This gives a product which is composed of a large number of linear channels and in which the channel walls have pores whose mean diameter is not more than about 3.5 .mu.m. Formation of the pores takes place during the final firing process and therefore cannot be influenced to any great extent. Furthermore, the preparation and shaping of the starting material is expensive and inconvenient. In addition, the linear, elongated channel structure results in unfavorable flow behavior.